A programmable logic integrated circuit has a feature in that various logic circuits can be reconfigured by rewriting internal set information. Thus, the programmable logic integrated circuit is used in a wide range of fields, such as creation of a prototype, image processing, and communication. In addition, in PTL 1 and PTL 2, a technique that can reduce the chip area and the power consumption by substituting a memory cell and a switching part of a crossbar switch used in a programmable logic integrated circuit with a resistive switching element is proposed.
Examples of the resistive switching element include ReRAM (Resistance Random Access Memory) using a transition metal oxide and NanoBridge (registered trademark) using an ion conductor. In PTL 1 and PTL 2, a resistive switching element using movement of metal ions and an electrochemical reaction in a solid (ion conductor) where ions can freely move by application of an electric field or the like is disclosed.
FIG. 2A, FIG. 2B, and FIG. 2C respectively illustrate a structure of a resistive switching element (RE), a symbolic expression of the resistive switching element (RE), and an operation method for changing the resistance of the resistive switching element (RE). As illustrated in FIG. 2A, the resistive switching element (RE) is composed of an ion-conducting layer (IC), and a first electrode (TR[1]) and a second electrode (TR[2]) that are provided on the opposite surfaces in contact with the ion-conducting layer (IC). Metal ions are supplied from the first electrode (TR[1]) to the ion-conducting layer (IC), and metal ions are not supplied from the second electrode (TR[2]). As illustrated in FIG. 2C, the resistance value of the ion conductor is changed by changing the applied voltage polarity, and ON/OFF is switched by controlling the conduction state between the two electrodes.
As illustrated in PTL 2 and FIG. 3A, a switch cell is composed of two resistive switching elements (RE1, RE2) and one transistor (Tr.). FIG. 3B is a symbolic expression of FIG. 3A focusing on terminals. As illustrated in FIG. 4A, in a crossbar switch, the switch cell is arranged in the vicinity of each of cross-points between wires (RV) in the vertical direction and wires (RH) in the horizontal direction. In addition, when turning ON/OFF a resistive switching element in the vicinity of a certain cross-point, in order to prevent erroneous writing (disturb) into a resistive switching element present in the vicinity of a different cross-point, the switch cell is also connected to two wires (SV, GH) for controlling a selection transistor (Tr.).
As illustrated in FIG. 4B, the crossbar switch takes the form in which at least four types of wires (RV, RH, SV, GH) pass in the vertical or horizontal direction. From the viewpoint of writing selectivity of the cell, the wire SV and the wire GH for switching, and the wire RV and the wire RH need to be skew each other, and, for example, are perpendicular as in FIG. 4B. The transistor in the switch cell is formed on a silicon substrate, and the resistive switching elements are formed in wiring layers.
The resistive switching elements can be the minimum size in the design rule or less. In addition, since ON/OFF switching at low voltage and low current has become possible by improvement in the element performance, the gate width of the selection transistor can be made small. Thus, the size of the entire crossbar switch has been defined by the four wires for the connection to the switch cell, and the occupation area necessary for the connection between the elements and the wires rather than the occupation area of the switch cell.
As illustrated in PTL 2 or PTL 3, the crossbar switch is used in a signal selection block (IMUX) of an input pre-stage into a look-up table (LUT) of a programmable logic integrated circuit, and a switching block (SMUX) for changing a routing path of a signal among a plurality of LUTs arranged in a programmable logic circuit.